This invention relates to noise insertion arrangements for digital speech transmission systems and, more particularly, to noise insertion arrangements for use in digital echo suppressors.
Noise insertion arrangements are useful in a variety of speech transmission systems in which the receive or incoming transmission path to a subscriber terminal is momentarily interrupted or disabled, such as for echo suppression purposes. The transmission facility typically contributes significantly to the overall background noise normally heard by the subscriber. Thus when the receive path is disabled by operation of an echo suppressor or other apparatus, a significant drop will occur in the noise level received by the subscriber, perhaps sufficient to cause him to believe that the transmission facility has failed or been disconnected. Ideally, the subscriber should be completely unaware of the operation of the echo suppressor or other apparatus. Toward this end, it is known to insert a noise signal on the path to the subscriber to substantially offset the drop in received noise level when the receive path is disabled. A limiter and frequency doubler arrangement for eliminating such noise fluctuations in an analog echo suppressor is disclosed, for example, in E. W. Holman-J. E. Unrue, Jr. U.S. Pat. No. 3,471,657 issued Oct. 7, 1969.
In digital transmission systems, the voice and other signals on the receive transmission path appear as digitally encoded words. Speech transmission over the receive path is typically disabled, such as for suppression of echoes, via connection of digital attenuator circuitry into the transmission path. See, for example, R. E. LaMarche-C. J. May, Jr. U.S. Pat. No. 3,673,355 issued June 27, 1972 and U.S. Pat. No. 3,823,275 issued July 9, 1974. As mentioned in the LaMarche-May U.S. Pat. No. 3,673,355, the digital attenuator circuitry may include a source of low-level noise to be extended to the subscriber while his receive path is disabled. Alternatively, W. L. Montgomery U.S. Pat. No. 3,699,273 issued Oct. 17, 1972 discloses an arrangement which blocks (or zeroes) all incoming code word bits except the least significant bits, which are allowed to pass through the attenuator circuitry unaltered, thereby providing a low-level quasi-random signal on the path to the subscriber during interruption of the receive path.
Limitations encountered with the Montgomery arrangement relate principally to the difficulty in achieving satisfactory signal levels to offset the noise drop during receive path interruption, particularly where the normally received noise level is of significant magnitude. Obviously the signal level can be increased by allowing additional bits of the incoming code words to pass unaltered through the attenuator circuitry. However, as two or more incoming bits are passed to the subscriber, the intelligibility of the incoming signal tends to be distinguishable and therefore objectionable. Of course, an independent noise source can be employed to achieve any desired signal level during receive path interruption, but with attendant disadvantages related to cost and complexity.